Public telecommunications networks were originally developed to provide voice communications between subscriber terminals. In recent years traditional wire has been replaced by fibre optics for long distance communications. With the expansion of such networks from the traditional telephone or POTS service into the provision of a variety of digital services, there is now a need to provide improved subscriber access to the network. Presently data traffic operates at frequencies below 5 KHz. Increasing amounts of data traffic are beginning to saturate the available channels and it is becoming apparent that higher operating frequencies are required to handle the increasing amounts of data, for example, from video on demand networks, direct digital broadcasts, and high capacity links for computers.
Typically subscribers are coupled to a telecommunications network with a twisted pair wire loop commonly referred to as the subscriber loop. This twisted pair connects to a node associated with an optical distribution network which can be some 1000 m away from the subscriber. Such an arrangement is depicted in FIG. 1. The first 950 m or so of cabling 12 from a junction box 10 is covered underground with trunking containing many twisted pairs of underground feeders 14, and the final 50 m or so of wire from a distribution point 16 to a subscriber's installation is normally above ground and consists of an individual twisted pair 18 to each house 20. Thus, for the last length of cable, the wires are ordinary telephone wires. If a subscriber's equipment is located proximate to a source of radio interference, for example transmissions from a radio amateur, then signals can be picked up very strongly on the two wires.
Typically, in twisted pair copper loop networks, signals are transmitted in differential mode; that is to say, the signals are transmitted along both wires and any Radio Frequency Interference (RFI) will be picked up by both wires approximately equally with the wanted signal being determined by the differential signal between the two wires, at the receiver. Since high speed data is transmitted in this fashion, there should be no transmission problems in such an arrangement. However, there may be present a certain amount of breakthrough between the common mode and the differential mode and some interference leaking through differentially, even though the interference is predominantly received in common mode. Differential interference signals may be up to 30 dB less than the common mode interference signal, but this can cause problems if it is strong enough to distort the output of the associated analogue-to-digital converter, ADC, in addition to providing an increase in the background interference resulting in a corresponding decrease in the output carrier-to-noise ratio, CNR.